In recent years, the amount of communications traffic has increased accompanying the spread of mobile phones known as smart phones and information communication terminal apparatuses known as tablet terminals. In order to respond to the traffic increase, a radio communication system based on Long Term Evolution (LTE) adopting Orthogonal Frequency Division Multiple Access (OFDMA) as the wireless accessing scheme is being developed. OFDMA is a wireless accessing scheme which has a frequency usage efficiency higher than that of Code Division Multiple Access (CDMA), which is tolerant of noise, resulting in less disconnection during calls. Because of this, LTE can be considered to be a standard for a radio communication scheme that realizes an increase in the capacity of radio communication systems and can also be considered to be a standard for a radio communication scheme that is capable of responding to increases in traffic.
Also, in order to respond to increases in traffic, development of heterogeneous networks in which macro cells and small cells are both used is discussed. FIG. 1 illustrates exemplary types of cells.
A cell is a section in a communication area that is covered by one base-transceiver station or by one remote-radio head (RRH) in a remote base station. A remote base station is a base-transceiver station that is also known as a remote installation base station, and includes a base band unit (BBU), which executes processes of base band signals etc., and at least one remote-radio head (RRH) for transmitting and receiving radio signals via an antenna. Cells are classified into macro cells or small cells in accordance with the sizes of communication districts that are covered (covered scope). A macro cell is a cell whose communication district is large, and has a cell radius of 35 kilometers or smaller in the example illustrated in FIG. 1. A small cell is a cell whose communication district is small, and has the cell radius of 2 kilometers or smaller in the example illustrated in FIG. 1. Also, as illustrated in FIG. 1, small cells can be classified further into micro cells (with a cell radius of 2 kilometers or smaller, for example), pico cells (with a cell radius of 200 meters or smaller, for example) and femto cells (with the cell radius of several tens of meters or smaller, for example).
Small cells have communication districts smaller than those of macro cells, and accordingly the number of mobile phones or information communication terminal apparatuses existing in a small cell may be small. As a result of this, the use of small cells suppresses the traffic per unit area and makes it easy to secure a high cell throughput. Therefore, according to a heterogeneous network having small cells formed in a district in which mobile phones or information communication terminal apparatuses may be concentrated in addition to having conventional macro cells with large communication districts, it is possible to increase the capacity of a radio communication system in response to traffic increases that can occur locally.
However, in a heterogeneous network as described above, the opportunities for a handover to occur, which occurs when a mobile phone or an information communication terminal apparatus moves from one cell to another, may increase in comparison with a communication network in which the communication area has only macro cells. FIG. 2 illustrates a handover that may occur in a heterogeneous network. In the example illustrated in FIG. 2, four small cells SC-1 through SC-4 are included in macro cell MC. Mobile radio apparatus MS illustrated in FIG. 2 is an example of a mobile phone and an information communication terminal apparatus described above.
When, as illustrated in FIG. 2, a user carrying a mobile radio apparatus MS that is conducting communications has moved, as depicted by a thick arrow, a handover (1) from macro cell MC to small cell SC-1 and a handover (2) from small cell SC-1 to macro cell MC may occur. Next, a handover (3) from macro cell MC to small cell SC-2, a handover (4) from small cell SC-2 to macro cell MC may occur. In a similar manner, a handover (5) through a handover (8) occur subsequently. As described above, in a heterogeneous network in which small cells are arranged in a macro cell, there are more opportunities for a handover to occur than there are in a communication network in which the communication area has only macro cells. Increases in opportunities for a handover to occur may lead to an increase in the risk of call losses. Therefore, in order to suppress the risk of call losses, it is preferable that opportunities for unnecessary handovers be suppressed as much as possible.
Regarding methods of switching channels of mobile communications, the following techniques are known. Specifically, it is determined whether a personal mobile phone is moving at a high speed or low speed on the basis of the degree by which the electric field strength of the communicating channel decreases. When it is determined that the personal mobile phone is moving at a high speed, the radio base station having an electric field strength equal to or higher than a specified value and having the electric field strength increasing the most sharply is selected so as to switch the channel.